The complex process of T-cell activation and proliferation is based on diverse interactions such as antigen presentation, cell-cell contact and soluble immune mediators e.g., cytokines or lymphokines. Many of these interactions are mediated in T-cells through surface receptors. T helper cells, for example, require for activation both the presentation of an antigen by an antigen presenting cell (APC) in association with major histocompatibility complex (MHC) and a secondary signal. The secondary signal may be a soluble factor or may involve an interaction with another set of receptors on the surface of T-cells. Antigen presentation in the absence of the secondary signal, however, is not sufficient to activate T helper cells.
The CTLA-4/CD28/B7 system is a group of proteins involved in regulating T-cell proliferation through this secondary signaling pathway. The T-cell proliferative response is controlled by the interaction of the B7 family of proteins, which are expressed on the surface of APCs, with CTLA-4 (cytotoxic T lymphocyte antigen #4) and CD28.
The B7 family of proteins is composed of structurally related glycoproteins including B7-1, B7-2, and B7-3 (Galea-Lauri et al., Cancer Gene Therapy, v. 3, p. 202-213 (1996); Boussiotis, et al., Proc. Nat. Acad. Sci. USA, v. 90, p.11059-11063 (1993)). The different B7 proteins appear to have different expression patterns on the surface of antigen presenting cells. For example B7-2 is constitutively expressed on the surface of monocytes, whereas B7-1 is not, although B7-1 expression is induced in these cells when the cells are stimulated with interferon gamma (IFN-.gamma.). The different expression patterns may indicate a different role for each of the B7 family members. The B7 proteins is believed to be involved in the events relating to stimulation of an immune response by its ability to interact with various immune cell surface receptors. It is believed, for example, that B7 plays a role in augmenting T-cell proliferation and cytokine production through its interaction with the CD28 receptor.
CD28, a homodimeric glycoprotein having two disulfide linked 44-kd subunits, is found on 95% of CD4.sup.+ and 50% of CD8.sup.+ cells. Studies using monoclonal antibodies reactive with CD28 have demonstrated that CD28 is involved in a secondary signal pathway in the activation of T-cell proliferation. Antibodies which block the interaction of CD28 with its ligand have been found to inhibit T-cell proliferation in vitro resulting in antigen specific T cell energy. (Harding et al., Nature, v. 356, p. 607 (1991)).
Recently a T-cell surface receptor protein, CTLA-4, having approximately 20% sequence homology to CD28 was identified. Although CTLA-4 is not endogenously expressed on T-cell surfaces, its expression is induced when CD28 interacts with B7 on the surface of an APC. Once CTLA-4 is expressed on the surface of the T-cell it is capable of interacting with B7.
Several groups have hypothesized that CTLA-4 and CD28 might have opposing effects on a T-cell and that CTLA-4 and CD28 might compete for binding of B7. (Krummel et al., International Immunology, v. 8, p.519-523 (1995); Galea-Lauri et al., Cancer Gene Therapy, v. 3, p. 202-213 (1996)). When a T-cell is presented an antigen by APC and B7 interacts with CD28 on the T-cell surface, a secondary signal is created which stimulates the T-cell to proliferate. When, however, B7 interacts with CTLA-4 the secondary signal is not created. It is still unclear whether the interaction of CTLA-4 with B7 initiates an inhibitory signaling pathway to prevent the cell from proliferating or whether the interaction of CTLA-4 with B7 simply acts to reduce the amount of B7 available for binding to CD28. In either case, it appears that CTLA-4, CD28, and B7 each play an important role in the intricate regulation of T-cell proliferation.